The present invention relates to a Doppler log, or vehicle movement measuring and indicating device, having transmitting and receiving units arranged on two intersecting measuring axes which are both parallel to a plane defined by the longitudinal and transverse axes of the vehicle for the emission of focused sound energy in directions which are inclined along the measuring axes by known, and possibly mutually different, emission angles and which are symmetrical to the normal to the measuring axes.
The transmitting units of such devices operate with known transmitting frequencies, which are possibly different for the different directions. For receiving the reflected component of the sound energy from the various directions, each receiving unit is connected to a Doppler frequency analyzer and a ratio former which is connected with the transmitting unit for the same direction and whose output signal represents the quotient of the Doppler frequency divided by the transmitting frequency.
It is known to determine the speed of a vehicle by means of a Doppler log. For this purpose, U.S. Pat. Nos. 3,603,920 and 3,691,513, for example, disclose the emission of a beam of sound energy at a known transmitting frequency f.sub.s and at an emission angle of .alpha. with respect to the horizontal. This energy is reflected by the ground underneath the vehicle, i.e. for ships the sea bottom, and the reflected components of the sound energy are received again with their frequency shifted with respect to the transmitting frequency f.sub.s by the Doppler frequency f.sub.D. The speed v of the vehicle is calculated from the Doppler frequency f.sub.D according to the following formula: EQU f.sub.D =2V/C.multidot.f.sub.s .multidot.cos .alpha.,
where C is the speed of sound in the medium traversed by the beam.
When a Doppler log is used to navigate a vehicle it is of great importance for the vehicle operator to be able to depend at any time on the accuracy of the Doppler log and for malfunctions to be indicated at once. A malfunction exists, for example, if a school of fish crosses the path of the ship and prevents accurate measurement of the speed of the ship with respect to the ground. Such a malfunction is of very short duration, and can therefore not be prevented or compensated for by testing the circuit with the aid of test instruments outside of the actual measuring operation.
It is of particular importance to have an indication of the accuracy of the momentary measuring result of the Doppler log if the Doppler log is used in a navigation system together with a compass to determine the course with reference to the north, whereby integration of the speed over successive time intervals of identical length furnishes indications of successive sections of the course of the vehicle. This is disclosed, for example, in U.S. Pat. No. 3,849,636.
The measuring results produced by the Doppler log could here be checked by means of some other navigation system which may possibly be available on board, e.g. a radio navigation system. However, a comparison of the speed indication provided by the Doppler log via associated navigation with sections of the course traversed within given time periods, as determined with the aid of additional navigation systems, is time consuming and complicated. Moreover, this would determine only coincidence in principle of the total systems within the confines of the given measuring tolerances, but would not be an unequivocal indication of the correctness and accuracy of the momentary speed indication.